The present invention relates to optical elements for the fabrication of diffraction gratings. More particularly, it relates to a non-uniform or apodized phase mask used for recording Bragg gratings or other high spatial frequency grating structures into an optical medium, the gratings having an apodized profile.
Already known in prior art, there is a uniform phase mask, which is a phase diffraction grating usually recorded in photoresist and then etched in a bulk substrate as a one-dimensional periodic surface-relief pattern. Phase masks are generally used in transmission to record Bragg gratings in the core of photosensitive optical fibers or on optical waveguides. The use of a phase mask significantly decreases the coherence requirements on the writing laser in comparison to holographic direct writing techniques. Ultraviolet (UV) light is usually used to imprint a Bragg grating into fibers or in photoresist, and therefore the phase mask material is preferably transparent to UV illumination.
The phase mask technique is described in U.S. Pat. No. 5,367,588 (HILL et al.). Hill discloses a method of fabricating Bragg gratings in the interior of a photosensitive optical waveguide comprising disposing a silica glass phase grating mask adjacent and parallel to the optical waveguide, and applying a single collimated light beam through the mask to said medium. The grating on the mask is so designed that the power in the transmitted zero diffraction order beam is minimized (in general to less than 5% of the light diffracted by the mask) and the diffracted power in the plus and minus first transmitted diffraction orders is maximized (each typically contain more than 35% of the incident light) over the entire grating surface. When illuminated at normal incidence with coherent or partially coherent light, the phase mask produces in its near-field a self-interference between the first order diffracted beams. This interference fringe pattern will have a spatial frequency that is twice the spatial frequency of the phase mask, and will be photo imprinted in the photosensitive medium placed in close proximity of the phase mask grating.
A variation of the above phase mask geometry, which uses off-normal illumination and the interference of the transmitted zero and minus-first diffraction orders, is also possible with small period phase masks and is disclosed in U.S. Pat. No. 5,327,515 (ANDERSON et al.) and U.S. Pat. No. 5,413,884 (KOCH et al.). In this case, the phase mask and the generated interference pattern have the same periodicity. If the phase mask period xcex9 has a size within the interval 0.5xcex less than xcex9 less than 1.5xcex, where xcex designates the readout wavelength in the medium, the phase mask generates, at incidence angles close to the Bragg angle, only the transmitted zero and minus-first diffraction orders. Since no diffraction orders have to be suppressed, the fabrication tolerances for the grating shape are less severe compared to the previous approach. This off-axis geometry has been proposed in the Anderson patent for the recording of Bragg gratings in an optical medium, and the Koch patent discloses a method of making such a mask.
Traditional phase masks comprise a uniform one-dimensional grating structure of constant period and relief depth, allowing for a constant diffraction efficiency of the mask. This generates a uniform periodic modulation of the refractive index in the photosensitive medium. The resulting device acts as a Bragg filter. Typically the Bragg spectral response will be a narrow-band reflectivity peak of nearly 100% at the design wavelength, accompanied by a series of sidelobes at adjacent wavelengths. The calculated reflectivity spectrum of a uniform Bragg grating is shown in FIG. 1 (prior art). It is important to reduce the reflectivity of the sidelobes in the Bragg response in devices where cross-talk cannot be tolerated, such as in devices for wavelength demultiplexing applications. Bragg gratings with reduced sidelobes in their spectral response can be achieved by recording an index modulation with a spatially varying modulation amplitude (see for example M. Matsuhara and K. O. Hill, Appl. Opt., 13, pp. 2886-2888 (1974) and I. Bennion et al., xe2x80x9cUV-written in-fibre Bragg gratingsxe2x80x9d, Tutorial review, optical and Quantum Electronics 28, pp. 93-135 (1996)). This generates a Bragg grating with a spatially varying coupling efficiency, which is called apodized. The condition for an efficient sidelobe suppression is that the modulation level decreases continuously to zero at both limits of the Bragg grating. Appropriate apodization profiles for this task are for example Gaussian, Blackman or Hamming functions or one period of the cos2 function (see for example D. Pastor et al., Journal of lightwave Technology, vol 14, no 11, pp. 2581-2588 (1996)).
Different approaches have been investigated to realize Bragg gratings generating a spectral response without sidelobes, with or without the use of a phase mask. Most of these approaches are variations based on three methods which will be described and discussed below.
The first of these methods concerns the use of a beam-profile shaping filter. In this approach, the intensity profile of the recording beams is appropriately shaped by means of absorptive, diffractive or Fabry-Perot apodizing filters, in order to generate the desired index modulation in the recording medium. Beam-shaping filters can be placed in the single beam incident on a uniform phase mask or in the two recording beams required for direct holographic writing in the fiber. However, with the beam-shaping filter technique, two successive recording steps with different filters have to be applied, in order to generate a narrow-bandwidth spectral Bragg response without sidelobes. The first step produces the periodic index modulation profile, whereas the second step compensates variations of the average refractive index in the recording medium, which is necessary to generate a single reflected Bragg wavelength (B. Malo et al. xe2x80x9cApodised in-fibre Bragg grating reflectors photo imprinted using a phase maskxe2x80x9d, Electr. Lett. Feb. 2, 1995, Vol. 31, No. 3, pp. 223-224).
The main drawback of the beam-shaping filter technique is that it is time consuming, difficult to use and therefore not appropriate for industrial production. It implies having to perform two exposure steps. The first exposure with apodized beams will induce the desired index modulation profile, but without a second exposure the local average refractive index will also vary, introducing an unwanted chirp. It is therefore necessary to expose a second time with a compensated beam to equalize the average refractive index. This second exposure step needs very precise alignment with respect to the first exposure. In addition, it is difficult to calibrate, since the response of the recording medium changes after the first exposure. Therefore, the final response of the Bragg grating is difficult to control in practice.
A second approach to write a Bragg grating having a reflection response with reduced apodized secondary maxima is the scanning beam technique. Variable exposure energies can be achieved by scanning focused beams or a slit mask in the recording beams with respect to the recording medium. The exposure energy is locally controlled by varying either the scanning speed or the optical power of the recording beams. The technique can be applied to recording with a single uniform phase mask or to direct holographic writing. As with the apodizer technique, two successive recording steps are required to generate a selective apodized Bragg grating (see J. Martin and F. Ouellette, xe2x80x9cNovel writing technique of long and highly reflective in-fibre gratingsxe2x80x9d, Electr. Lett., 30, 811-812 (1994)). An interesting implementation of this technique has been proposed in M. J. Cole et al. xe2x80x9cMoving fibre/phase mask-scanning beam technique for enhanced flexibility in producing fibre gratings with uniform phase maskxe2x80x9d, Electr. Lett., 31, 1488-1490 (1995). It is shown that pure apodization is achieved by either applying a variable spatial oscillation to the fiber or by slowly moving it relative to the uniform phase mask as the writing beam is scanned. This technique modifies locally the contrast of the recorded interference pattern but does not affect the average recording intensity. Therefore, only a single scanning exposure is required. Another variation is to apply a symmetric longitudinal stretching of the fiber around the center of the grating while the grating is being written, as discussed in R. Kashyap, A. Swanton and D. J. Armes, xe2x80x9cSimple technique for apodising chirped and unchirped fibre Bragg gratingsxe2x80x9d, Electr. Lett., 32(13), June 1996, pp 1226-1228.
The scanning beam technique is however a sequential recording process which is time-consuming and therefore not appropriate for the production of apodized spectral response Bragg gratings. In the case of two exposure steps, the same arguments as for the beam-shaping filter technique apply and are responsible for a difficult process control. The improved scanning technique of the above mentioned reference from M. J. Cole et al. requires sophisticated automated positioning equipment which is able to apply a controlled dither to the fiber or the uniform phase mask, while the writing beam is scanned. For each position of the scanning beam, the movement applied to the phase mask or fiber must be carefully controlled to have a specific frequency and amplitude, in order to generate the desired variation of the local diffraction efficiency. The same difficulty arises in the stretching technique of Kashyap et al. where the stretch induced by two piezo-translators must be carefully calibrated. In practice the resulting apodized spectral response of the recorded Bragg grating is very difficult to predict.
A final approach to sidelobes reduction is the use of a non-holographic apodized phase mask. This technique has been proposed in the paper from J. Albert et al., xe2x80x9cApodisation of the spectral response of fibre Bragg gratings using a phase mask with variable diffraction efficiencyxe2x80x9d, Electr. Lett. Feb. 2, 1995, Vol. 31, No. 3, pp. 222-223. It applies a single recording step with an apodized phase mask of variable diffraction efficiency. The apodized phase mask with variable diffraction efficiency is fabricated by direct writing in silica with a focused ion beam followed by differential wet etching. The first orders diffraction efficiency apodization can be performed by varying spatially the grating relief depth, the line width or both parameters at the same time.
The apodized phase mask technique is the most appropriate technique for the production of apodized grating structures. It generates the apodized grating with a single exposure which enables high reproducibility and short fabrication times. However, the apodized phase mask structure proposed in the Albert et al. reference suffers from several severe drawbacks.
The proposed apodized phase mask is fabricated by applying direct-write technologies which can use either a focused electron, ion or laser beam as source. The direct-write method has the advantage of being flexible because the grating is written one line at a time, or one writing field at a time. Thus, the implementation of complex apodization profiles is straight forward. Preferentially, the variable diffraction efficiency of the phase mask grating is realized by modulating the line width of the grating structure. Then, a binary surface relief is obtained in resist which can be transferred in the substrate by standard binary selective etching technology. Unfortunately, direct writing is a sequential fabrication process and the realization of large surface phase masks, as required for fiber Bragg grating printing and photolithography on wafers for batch processing of optical waveguide devices, is time consuming and expensive. Furthermore, direct writing introduces position errors in the ideal grating structure which influence strongly the spectral response of uniform and apodized Bragg gratings. FIG. 2 (prior art) shows the typical reflectivity spectrum of an apodized fiber grating realized with a direct electron-beam written phase mask (direct-write fabrication errors are discussed in T. Kjellberg et al., Jour. Lightwave Technol., 10, pp. 1256-1266 (1992), A. Swanton et al., xe2x80x9cUse of e-beam written, reactive ion etched, phase masks for the generation of novel photorefractive fibre gratingsxe2x80x9d, Microelectronic Engineering 30, 509-512 (1996), and M. J. Verheijen, xe2x80x9cE-beam lithography for digital hologramsxe2x80x9d, Jour. of Mod. Opt., 40, 711-721 (1993)). Two different error contributions can be distinguished. As the writing beam changes from one writing field to another, misalignment problems cause overlap or spacing between the exposed fields. These position errors are called stitching errors and contain in general a systematic contribution. Systematic stitching errors are intolerable for apodized Bragg gratings, since they produce important sidelobes in the spectral Bragg response based on Fabry-Perot interference effects (FIG. 2). In addition, random position errors distributed stochastically along the grating lines of the phase mask are generated. As shown in FIG. 2, random errors lead to a considerable increase of the noise-level outside of the Bragg peak.
Since wavelength demultiplexing devices in optical telecommunication require noise-free Bragg responses without any sidelobes, high-quality apodized phases masks cannot be realized by direct-write technologies. Therefore there is presently a need for an improved method of making apodized phase masks
Holography is a well known alternative method for fabricating phase masks. In this method, the sinusoidal interference pattern of two coherent optical beams is first recorded in photoresist as a surface-relief hologram which is then transferred in the substrate medium by etching. Holography is a parallel writing process and is therefore appropriate to the realization of large surface area phase masks. In addition, optical,wavefronts can be generated very accurately over large surfaces by using conventional lenses or mirrors. As a result, the holographically recorded grating pattern is free from any positioning errors and a sub-Angstrom grating precision on the grating period over the entire grating surface is conventionally achieved. Therefore, holography is a promising fabrication technology for apodized phase masks generating Bragg gratings with ideal, noise-free spectral responses.
However, generating a phase mask of spatially varying diffraction efficiency with holographic recording is not an easy task. As already stated above, an efficiency modulation can be achieved by varying the grating relief depth or the line width. A precise local variation of the grating line width, as achievable with direct writing, is difficult to implement holographically and therefore not recommended for commercial applications.
Therefore, the efficiency variation of the apodized phase mask has to be realized by relief depth modulation. The fabrication of such grating structures in photoresist by direct holographic recording with a locally varying exposure energy has been demonstrated in A. Mitreiter et al., xe2x80x9cApodized diffraction grating as outcoupling element for 1.06 xcexcm Nd:YAG laserxe2x80x9d, OSA Technical Digest Series Vol. 11, Diffractive Optics: Design, Fabrication, and Application, Rochester, 1994, pp. 282-285. The considered apodized grating in this reference relates to reflective output coupling elements in a laser resonator, which is a complete different application from the apodized phase mask. This existing approach for the realization of apodized phase masks presents two major drawbacks. In order to generate the varying modulation depth without distortion, photoresist development and substrate etching with rather linear process characteristics have to be applied. Linear processes are not standard in microfabrication technologies: they are difficult to calibrate, not reproducible and therefore not ideal for production. Furthermore, a linear development and etching process will lead to a sinusoidal surface relief. With the sinusoidal relief in fused silica, the phase mask will only achieve for normal incidence a maximum first-order diffraction efficiency of 30% and have a zero-order contribution. of 15% efficiency. These are poor performances compared to a rectangular surface relief with 40% first-order efficiency and complete extinction of the zero order. The sinusoidal phase mask would require much longer exposure times for the recording of efficient Bragg gratings, and at the same time would modify significantly the average refractive index which results in poor control over the final Bragg wavelength.
Looking to the field of optical elements in general, U.S. Pat. No. 4,936,665 (Whitney) and U.S. Pat. No. 5,587,815 (Sato et al) may finally be mentioned. Whitney discloses high resolution imagery systems and methods including a blazed transmission grating having grating rings of low bending power defined by multiple plateaus. Sato et al discloses a binary phase modulation device provided with protrusions of varying width to generate a diffraction efficiency distribution.
On account of the disadvantages displayed by the existing methods used to fabricate apodized Bragg gratings, there is presently a need for a new fabrication method which will avoid their drawbacks, combine their main advantages, and lead to dramatically improved results in the spectral response of the recorded apodized Bragg gratings.
An object of the present invention is to propose a phase mask and a method for fabricating a phase mask which will satisfy the above-mentioned needs. More particularly, the present invention concerns a phase mask for modulating a collimated light beam passing therethrough, the light beam being diffracted to photoinduce by self-interference a refractive index profile in a photosensitive optical medium, the phase mask comprising:
a substrate having an outer surface provided with a plurality of parallel grating corrugations, the grating corrugations having a non-uniform relief depth across the outer surface, the phase mask being characterized in that the non-uniform relief depth is maximum at a center of the grating corrugations and decreases continuously to zero at opposite ends of said grating corrugations for photoinducing an apodized refractive index profile in the photosensitive optical medium, the non-uniform relief depth being defined by a variable thin film layer of variable thickness overlaying the substrate.
In a first preferred embodiment of the invention, the grating corrugations comprise a plurality of grooves etched into the variable thin film layer. The substrate may comprise an etch stop layer lying under the variable thin film layer.
In a second preferred embodiment of the invention, the grating corrugations comprise a plurality of grooves etched into the substrate, the variable thin film layer being deposited into the grooves.
Another object of the present invention is to propose a method for making a phase mask having a non-uniform grating profile. A first preferred version of the method comprises steps of:
a) depositing a variable thin film layer having a variable thickness on a substrate;
b) depositing a photoresist layer on the variable thin film layer;
c) recording a grating pattern in the photoresist layer;
d) etching the grating pattern through the photo-resist layer and into the variable thin film layer, the variable thickness of the variable thin film layer generating the non-uniform grating profile; and
e) removing residual photoresist remaining on the variable thin film layer.
Another preferred version of the method for making a phase mask according to the present invention comprises steps of:
a) depositing a variable thin film layer having a variable thickness on a uniform phase mask, the uniform phase mask having an outer surface provided with a plurality of uniform parallel grating corrugations, the corrugations defining grooves and ridges so that a portion of the thin film layer is deposited on the ridges and another portion of the thin film layer deposited in the grooves, and
b) removing the portion of the variable thin film layer that is deposited on the ridges of the corrugations.
Preferably, this method comprises an additional step of making the uniform phase mask. This additional step comprises steps of:
i) depositing a photoresist layer on a substrate;
ii) recording a grating pattern in the photoresist layer; and
iii) etching the grating pattern through the photoresist layer and into the substrate to obtain the grating corrugation.
Also preferably, step b) of this method further comprises removing residual photoresist remaining on the corrugations.
A better understanding of the invention will be obtained by reference to the detailed description below in conjunction with the appended drawings.